I know that you can outwit defensive abilities before poison takes effect, but if there were are 4 io Jokers basing si Doom, and I outwit Doom's Imperv, do all 4 poisons take effect immediately? Or after the first poison gets through, do I have to immediately outwit Invuln, wait until 2 more poisons get through then outwit toughness for the final poison to take effect? I have 4 outwits, so it isn't a huge deal in this scenario, but it is always better to use 1 outwit instead of 3.

You, as the player who is taking the current turn, can decide what order to apply all your actions (including those free actions occuring at the beginning of the turn).

So you could use all the Poison at once... which would end up wasting three against the unOutwitted Invulnerability after the first knocks Doom off Impervious. Or you could choose to Outwit Impervious, deal one click of Poison, Outwit Invulnerability, deal another click of Poison, deal a third click, Outwit Toughness, and then deal a final click of Poison.

Quoted for truth.

So poison is stackable? I thought that defense had to be outwitted or the poisoner needs AP in order to get any poison through, like each person is dealing 1 damage and it is all getting absorbed.

No Poison is not stackable. Again:

So you could use all the Poison at once... which would end up wasting three against the unOutwitted Invulnerability after the first knocks Doom off Impervious. Or you could choose to Outwit Impervious, deal one click of Poison, Outwit Invulnerability, deal another click of Poison, deal a third click, Outwit Toughness, and then deal a final click of Poison.

Note, in the bolded section, that if Poison was stackable, one damage would get through the Invulnerability (3 - 2 = 1)... and it doesn't (and I never claimed it did). Instead, Doom would take no damage on the Invulnerability click (1 -2 < 0, 1 -2 < 0, and 1 -2 < 0).

OK just try to remember that you are still in the "Begining of your turn" Phase of your turn. And as such you can do any action that is possible in that phase at your discretion.
Joker 1 outwits then deals poison
Joker 2 Outwits then deals Poison
Twoface rolls for leadership
Riddler perplexes up his damage.
Joker 3 Deals Poison and outwits
Joker 4 Deals Poison.

So far you have not taken any NON-Free Actions so you are still in the begining of your turn phase.

I gotcha. for a second I was thinking that you would be wasting 3 of the poisons due to only one getting through after the stacking, but then that doesn't make sense either seeing that 2 would get through upon stacking...

I need sleep sometimes.

Just for clarity's sake, this is a wee bit misleading.

You can take a non-free action and still be in the beginning of the turn as long as the action in question specifies that it occurs at the beginning of the turn (example: gaining a power via ICWO, which is a power action that occurs at the beginning of the turn).

Also, there are some free-actions that cannot be part of the beginning of the turn phase like absorbing an object via nanobots

Also I'm pretty sure there are some free actions that don't necessarily take place at the beginning of turn and said beginning of turn phase.

Edit: Doh, too slow!

Yes, correct on both counts. I guess I should have said any action that precludes him from continuing to use poison, like making an attack or taking a move action or any action that cannot be activated in the begining of the turn. My example was overly simplified but stuck on the topic in which Chirocker requested an answer on.